JP2000169172A - Production of optical fiber preform - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a process for producing an optical fiber preform capable of efficiently producing a fluorine added optical fiber preform which is uniform in a concentration distribution of a diametral direction. SOLUTION: Hydrofluoric acid or hydrogen fluoride reacts with glass to form silicon tetrafluoride and water. Then, if soot 1 is held in a hydrogen fluoride atmosphere, the hydrogen fluoride diffused from the surface into the inside of the soot 1 reacts with the glass of the soot and produces the silicon tetrafluoride. The concentration of the hydrogen fluoride is, therefore, kept always low with respect to the ambient atmosphere in the arbitrary position within the soot 1. Accordingly, the degradation in the diffusion rate by the saturation of the hydrogen fluoride into the soot 1 may be prevented and the concentration distribution in the diametral direction is made uniform.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a method for manufacturing an optical fiber preform.

[0002]

After forming glass porous the (soot) was hydrolyzed as one of the manufacturing method of an optical fiber preform of SiCl _4, GeCl ₄ and the like of the raw material gas in an oxyhydrogen burner, a heat treatment There is a method of making the glass transparent. Such a method is called a VAD method (gas phase shafting method), an OVD method (external VAD method), or the like. One of the means for controlling the refractive index in these optical fiber preform manufacturing methods is the addition of fluorine. When fluorine is added to quartz glass, the refractive index decreases. Another additive that lowers the refractive index is boron.

However, since the amount of decrease in the refractive index per single concentration is larger for fluorine, the increase in loss due to addition can be suppressed by using fluorine.

At present, the fluoridation technique is applied to the production of various optical fibers such as a low loss single mode optical fiber, a dispersion shift optical fiber, and a dispersion compensating optical fiber.

Conventionally, the addition of fluorine is performed either during soot formation or soot sintering.

As a fluorine-containing gas, SF ₆ , Si
F ₄ , C ₂ F ₆ , CF _{4 and the} like are used. When fluorine is added at the time of soot formation, the fluorine-containing gas is hydrolyzed in a flame together with other raw material gases to form glass fine particles, and the glass fine particles are deposited on a quartz target to form a fluorine-containing soot. This fluorine-containing soot is heated at 1350 ° C. in an electric furnace to form a transparent vitreous glass, thereby obtaining a base material for an optical fiber. When fluorine is added at the time of soot sintering, a soot containing no fluorine is formed, and then sintering is performed in an atmosphere gas containing a fluorine-containing gas.

[0007]

By the way, in the method of adding fluorine during soot production, a part of the fluorine-containing gas is exhausted without reacting. Further, even when glass fine particles are formed by a reaction, there is also a case where fine particles are exhausted without being deposited as soot. Therefore, the utilization efficiency of the reaction gas is poor. In the method of adding fluorine during soot sintering, the fluorine-containing gas is added to the soot by diffusion from the surface. For this reason, the diffusion speed is different between the soot surface portion and the central portion, and a fluorine addition concentration distribution tends to occur in the soot radial direction.

Therefore, it is necessary to diffuse the fluorine-containing gas sufficiently inside the soot by performing sintering for a long time, or to promote the diffusion by sintering the soot in a fluorine gas atmosphere pressurized to 1 atm or more. Was. Therefore, there is a problem that it is necessary to use a large amount of expensive fluorine-containing gas in any method.

An object of the present invention is to solve the above-mentioned problems and to provide a method of manufacturing an optical fiber preform capable of efficiently manufacturing a fluorine-doped optical fiber preform having a uniform radial concentration distribution.

[0010]

In order to achieve the above object, a method for manufacturing an optical fiber preform according to the present invention is directed to an optical fiber preform which is made transparent by heat treatment to obtain an optical fiber preform. In the manufacturing method, after the glass porous base material is held in a hydrogen fluoride atmosphere, a heat treatment is performed to make the glass viscous.

In addition to the above-described structure, the method of manufacturing an optical fiber preform according to the present invention comprises subjecting a porous glass preform to a vacuum degassing process before a hydrogen fluoride atmosphere process.

In addition to the above structure, in the method of manufacturing an optical fiber preform of the present invention, the hydrogen fluoride atmosphere is preferably formed by bubbling hydrogen fluoride or hydrofluoric acid with a gas.

In addition to the above structure, in the method of manufacturing an optical fiber preform of the present invention, the hydrogen fluoride atmosphere is preferably formed by heating hydrogen fluoride or hydrofluoric acid.

A feature of the present invention resides in that the soot to which fluorine is added is kept in a hydrogen fluoride atmosphere, and then is transparently vitrified by sintering.

Hydrofluoric acid or hydrogen fluoride reacts with glass to form silicon tetrafluoride and water. Therefore, when the soot is kept in a hydrogen fluoride atmosphere, the hydrogen fluoride diffused from the soot surface to the inside reacts with the soot glass to produce silicon tetrafluoride. For this reason, the hydrogen fluoride concentration is always kept low relative to the surrounding atmosphere at an arbitrary position inside the soot. Therefore, according to the present invention, it is possible to prevent a decrease in the diffusion rate due to saturation of hydrogen fluoride in the soot, and the radial concentration distribution becomes uniform.

Also, hydrogen fluoride has a high diffusion rate because it has a smaller molecular weight than other fluorine-containing gases such as silicon tetrafluoride. Therefore, when hydrogen fluoride is used, the concentration distribution of hydrogen fluoride in the soot radial direction is small, and as a result, the concentration distribution of silicon tetrafluoride generated by hydrogen fluoride can be reduced.

By performing vacuum degassing before holding in a silicon fluoride atmosphere, the amount of gas that hinders the diffusion of hydrogen fluoride in the soot is significantly reduced. And fluorine can be efficiently added.

By bubbling these liquids with a gas (for example, Ar or the like), the evaporation of hydrogen fluoride can be promoted. A similar effect can be obtained by heating hydrogen fluoride or hydrofluoric acid.

[0019]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings.

FIG. 1 is a conceptual diagram showing an embodiment of an apparatus to which the method for manufacturing an optical fiber preform according to the present invention is applied.

In FIG. 1, reference numeral 3 denotes a bubbler which is heated by a heater 4, and a gas introduction pipe 6 is inserted near the bottom. The gas introduction pipe 6 is branched outside the bubbler 3 and connected to the three-way valve 8a. The outlet side of the three-way valve 8a is connected to one inlet side of the three-way valve 8b via a pipe. The suction port of the vacuum pump 5 is connected to the other inlet side of the three-way valve 8b, and the outlet side of the three-way valve 8b is connected to the bottom of the vertically disposed soot holding box 2 through a pipe. . A support bar 10 is horizontally mounted on the upper side in the soot holding box 2. The support rod 10 has a quartz target rod for core (hereinafter referred to as “target rod”).
The soot 1 is formed on the surface of the target rod 9. An exhaust pipe 7 is connected to an upper portion of the soot holding box 2, and the exhaust pipe 7 is provided with a shutoff valve 11.

[0022]

An embodiment of the present invention will be described below.

First, the core portion of the optical fiber is VAD
It is deposited by a method, and is heated to a temperature of 1350 ° C. in an electric furnace and transparent vitrification is performed (not shown). The cladding soot 1 is deposited outside the target rod 9 by an external VAD method.

FIG. 2 is a conceptual diagram of a soot deposition apparatus using an external VAD method.

The soot deposition apparatus includes a housing 15 and a housing 15.
A mechanism (not shown) for rotating and raising the target rod 9 therein, an oxyhydrogen burner 12 disposed below the housing 15, and a pipe 16 provided below the housing 15. .

With such a soot deposition apparatus, the raw material gas (silicon tetrachloride) is hydrolyzed in the oxyhydrogen burner 12 to form fine glass particles, which are deposited on the target rod 9 which moves upward while rotating.

Next, the external soot 1 is detached from the soot depositing apparatus, put into the soot holding box 2 shown in FIG. 1, fixed with the support rod 10, and then closed (not shown).
The shut-off valve 11 is closed, the three-way valve 8b is opened on the vacuum pump 5 side, and the inside of the soot holding box 2 is evacuated to 0.1 Torr. After that, the bubbler 3 side of the three-way valve 8a is opened, and the shutoff valve 11 is opened to perform bubbling. At the time of bubbling, 1 liter of argon gas is flown per minute in 49% hydrofluoric acid. The hydrofluoric acid is heated to 40 ° C. by the heater 4. The piping is also heated to 45 ° C. by the heater 4 to prevent hydrofluoric acid from aggregating in the piping (not shown). After 4 hours, bubbling is stopped, the bubbler 3 side of the three-way valve 8a is closed, and the hydrogen fluoride remaining in the soot holding box 2 is sufficiently evacuated, and then the soot 1 is taken out. The soot 1 thus taken out is heated to a temperature of 1450 ° C. and vitrified in an electric furnace (not shown) to obtain an optical fiber preform.

FIG. 3 is a diagram showing a radial refractive index distribution of an optical fiber preform manufactured by the apparatus shown in FIG.

The cladding region 14 around the core region 13 is a portion to which fluorine has been added. It can be confirmed that fluorine is uniformly added in the cladding region 14. Also,
The optical fiber preform was drawn to evaluate the transmission loss at a wavelength of 1.55 μm, which was as low as 0.21 dB / km.

As described above, according to the optical fiber preform manufacturing method, a fluorine-doped optical fiber preform having a uniform concentration distribution in the radial direction can be easily manufactured.

[0031]

According to the present invention, the following excellent effects are exhibited.

It is possible to provide a method of manufacturing an optical fiber preform capable of efficiently producing a fluorine-doped optical fiber preform having a uniform radial concentration distribution.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram showing an embodiment of an apparatus to which an optical fiber preform manufacturing method of the present invention is applied.

FIG. 2 is a conceptual diagram of a soot deposition apparatus using an external VAD method.

FIG. 3 is a diagram showing a radial refractive index distribution of an optical fiber preform manufactured by the apparatus shown in FIG. 1;

[Explanation of symbols]

 Reference Signs List 1 soot 2 soot holding box 3 bubbler 4 heater 5 vacuum pump 6 gas inlet pipe 8a, 8b three-way valve 9 target rod 10 support rod

Claims (4)

[Claims] 1. A method of manufacturing an optical fiber preform, wherein a glass porous preform is made transparent by heat treatment to obtain an optical fiber preform. And producing a transparent glass. 2. The method for producing an optical fiber preform according to claim 1, wherein the glass porous preform is subjected to a vacuum degassing treatment before being subjected to a hydrogen fluoride atmosphere treatment. 3. The method according to claim 1, wherein the hydrogen fluoride atmosphere is formed by bubbling hydrogen fluoride or hydrofluoric acid with a gas. 4. The hydrogen fluoride atmosphere is formed by heating hydrogen fluoride or hydrofluoric acid.
3. The method for producing an optical fiber preform according to 1.).